JumpThreading may convert selects into branch instructions,
in which case the condition needs to be frozen (as branch on
poison is immediate undefined behavior, unlike select on poison).
The necessary code for this is already in place, this just enables
the option.
Differential Revision: https://reviews.llvm.org/D125869
Duplicating instructions can lead to code size increases but using
a threshold of 3 is good for reducing code size.
Differential Revision: https://reviews.llvm.org/D72916